This invention relates generally to radiotelephones and radiotelephone systems and, in particular, to time division multiple access (TDMA) cellular radiotelephones or mobile stations, as well as to radio communications systems and networks.
In at least one type of TDMA radio communications system the start of a reverse TDMA frame on the uplink, i.e., from the mobile station (MS) to a base station (BS), is delayed by a fixed period of three timeslots from the start of a forward TDMA frame on the downlink, i.e., from the base station to the mobile station. After adding a propagation delay, which is compensated for by a timing advance (TA) parameter, one obtains the situation depicted in FIG. 1.
As may be appreciated, an increase of the TA induces a decrease of Trt (the time from the end of reception to the start of transmission.) However, this decrease of Trt can result in the occurrence of one or more problems. For example, then the mobile station is far enough from its serving base transceiver station, or BTS, TA can become larger than some maximum value expected by the wireless network. By example, in a Global System for Mobile Communications (GSM) network this maximum value corresponds to a cell radius of about 35 km, and therefore Trt can become less than an expected minimum value. This is significant, as the minimum value of Trt is used to fix the multislot capability of the mobile station and network.
Two parameters depend on Trt, as specified in a document entitled xe2x80x9cGSM 05.02: Digital cellular telecommunication system (Phase 2+); Multiplexing and multiple access on the radio pathxe2x80x9d. The two parameters are as follows:
Tta: For a type 1 mobile station (i.e., one that is not required to transmit and receive at the same time) this parameter specifies a minimum number of timeslots that will be allowed between the end of the previous transmit or receive time slot and the next transmit time slot, when a channel measurement is to be performed by the mobile station in between.
Ttb: For the type 1 mobile station this parameter specifies the minimum number of timeslots that will be allowed between the end of the last receive time slot and the first, next transmitted time slot, or between the previous transmit time slot and the next transmit time slot when the frequency is changed in between.
These two parameters are specified in numbers of timeslots. Thus, if TA is large enough the effective value (from the mobile station point of view) can be reduced by one time slot. However, it cannot be reduced by a value of two timeslots since TA is limited in such a way that this is not possible. In other words, the cells are not large enough.
By example, when a type 1 mobile station travels further than the maximum cell radius that is specified in GSM (35 km), its Trt is decreased to the point that the mobile station may not be able to transmit on some portion of its allocated uplink burst after receiving its downlink burst. This can be referred to as an overlapping case, and an example thereof, with the following allocations, is shown in FIG. 2. Assume for the case of FIG. 2 that the network is HSCSD and asymmetric, that downlink time slots 2 and 3 are used, and that time slot 2 is used on the uplink. When the mobile station is sufficiently far from its serving BTS, the increase in the TA results in an overlap of the uplink and downlink bursts. Therefore, one burst would be lost by type 1 mobile stations.
On the other hand, an increase in TA induces an increase in Ttr. Thus, no problem should arise with the associated parameters (Tra,Trb). Reference in this regard can be had to the above-noted document: xe2x80x9cGSM 05.02: Digital cellular telecommunications system (Phase 2+); Multiplexing and multiple access on the radio pathxe2x80x9d. The effective value of the parameters, from the mobile station point of view, cannot become lower than the minimum value expected.
It is known in the prior art to deal with large radius cells by forbidding traffic in consecutive time slots. However, this technique can reduce the capacity of the network by one half for the affected mobile stations, and does not support multislot operation, such as that specified for HSCSD or GPRS.
It is thus a first object of this invention to provide a method to enlarge the cell radius in a TDMA system, while avoiding problems resulting from the longer propagation delays.
It is another object and advantage of this invention to provided dynamic method of modifying at least one timing parameter to accommodate a mobile station in a large radius cell of a TDMA network.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed to effectively enlarge the cell radius in a TDMA based system in such a way that the timing advance (TA) parameter is modified by increasing the parameter range. The timing advance may be greater than one time slot and less than two time slots. Uplink and downlink overlapping is prevented by decreasing the time limit requirement from the end of reception to the start of transmission. A Trt value is allowed to decrease in the same respect as the TA parameter is allowed to be increased, when compared to the conventional GSM system, and a maximum additional decrease of the Trt value is one time slot. The propagation delay increase of cells having a larger radius is monitored in the base station, and the value of TA can be considered as well when mapping channels in the base station.
In an exemplary embodiment, and considering a GSM450 case, the radius of a cell can be larger than the conventional GSM radius (35 km). In order to cope with this increase in cell radius, the timing advance parameter is modified such that its effective range is increased. However, because of the greater range of the timing advance parameter, the operation of the TDMA mobile station may be adversely affected in such a way that bursts in the uplink (reverse link) and in the downlink (forward link) would overlap in time. An aspect of this invention is thus in providing a technique to make a multislot capability of the mobile station dynamic, thereby mitigating or eliminating this problem.
A method is disclosed for operating a wireless TDMA communications system. The method includes steps of monitoring a propagation delay of a mobile station; and when the propagation delay exceeds a predetermined limit, changing by a time slot unit a value of one or both of Tta1 and Ttb1, wherein Tta1 is a minimum number of time slots allowed between the end of a last, previous receive time slot and a first, next transmit time slot, when a channel measurement is to be performed in between, and wherein Ttb1 is a minimum number of time slots allowed between the end of a last, previous receive time slot and a first, next transmit time slot.